Langmuir probes have been widely used to analyze both laboratory and space plasmas. The basic principle of the Langmuir probe is to expose a conductor to the plasma, bias it relative to some reference potential (the platform potential) and measure the collected current. A representative response of a Langmuir probe is shown in FIG. 1.
A body in a plasma (having a plasma potential Vp) will experience a current as it is hit by the electrons and ions of the plasma. This changes its charge, which in turn affects the electron and ion currents. In a short time the body will acquire a charge so that the net current is zero. The equilibrium potential attained by a conductive body immersed in a plasma, such that the total current due to electrons and ions to the conducting surface sums to zero is known as the platform potential (Vf). The platform potential is also referred to as the spacecraft potential, payload potential, floating potential or payload floating potential. The plasma potential Vp, is the potential at which no fields/sheath exist between the probe surface and the surrounding plasma. A probe potential lower than Vp repels electrons, and a potential higher than Vp repels ions.
When the probe is negatively biased it will attract positive ions and repel electrons. At a sufficiently negative bias the positive ion current will dominate, this is called the ‘ion saturation region’ 1. When the probe is positively biased it will repel positive ions and attract electrons. At sufficiently positive bias the electron current will dominate and this is known as the ‘electron saturation region’ 2. At an intermediate bias, the current will be the sum of the ion and electron currents, this is known as the ‘retardation region’ 3.
The Langmuir probe is normally used in a voltage-sweep mode. Performing several measurements at different biases a current-voltage graph can be produced. This is then fitted to the theoretical equations for plasma, photoelectron and possibly other currents to extract plasma parameters.
For a better understanding on how turbulence in ionospheric plasma affects high frequency (HF) radio communication and Global Navigation Satellite Systems (GNSS) signals, a study of the driving processes which cause turbulence and structuring of the ionospheric plasma has to be carried out. One of the key parameters to understand turbulence in space plasma is the electron density which can be measured using a Langmuir probe.
Performing a Langmuir probe sweep takes time (on the order of 1 s). There are also several factors that can cause measurements to deviate from the ideal equations. It has been found that the swept Langmuir probe provides low accuracy ionospheric electron density measurements due to uncertainties in the determination of the spacecraft potential as well as the electron temperature. Also, a sample time of 1 s means that the resolution of the measurements is limited.
Thus, to overcome this problem it is known to use a spherical fixed bias Langmuir probe. However, the current collected by a spherical fixed bias probe depends not only on the electron density but also on the electron temperature and the spacecraft potential. As a result, the absolute electron density is very difficult to determine. There are further physical effects that complicate efforts to obtain absolute electron density measurements.
To overcome this problem it is known to provide a Langmuir probe system comprising two or more fixed-bias cylindrical probes which is known as a multi-Needle Langmuir probe (m-NLP). Each cylindrical probe is biased to a different potential in the electron saturation region and the probes are sampled simultaneously to obtain a measurement of the collected current of each probe. This device can be used to determine the absolute electron density without having to know the electron temperature and spacecraft potential (also referred to as the platform potential, payload potential or payload floating potential).
The m-NLP is able to measure electron density with a very high time resolution, which provides high spatial resolution of electron density measurements.
To perform the absolute electron density measurements using the m-NLP only an estimate of the spacecraft potential is necessary. Potentials well above the spacecraft potential can be used to ensure that the probes are operating in the saturation region where absolute electron density measurements can be taken.
The square of the probe current (Ic2) versus the probe potential (V) yields a straight line. The line gradient is proportional to the electron density squared. The absolute electron density can be calculated according to the following expression:
                              n          e                =                              K            ⁢                                          Δ                ⁡                                  (                                      I                    c                    2                                    )                                                            Δ                ⁢                                                                  ⁢                V                                                                        (        1        )            
where K is a constant equal to me/2q(q2rl)2 and me is the electron mass, q is the electron charge, r is the radius of the cylinder, I is the length of the cylinder, Δ(Ic)2 is the difference in collected probe currents and ΔV is the difference in probe bias between the two biased probes.